ROTATION OF CROPS. . 53 



land by means of ammonia taken up from the atmosphere 

 by the leaves of the crop, for nothing is known as to the 

 quantity of nitrogen which crops may thus acquire. 



It follows from what we have just stated that the benefits 

 resulting from the growth of a green crop in a rotation 

 are greater in proportion as its period of growth is longer, 

 and its roots deeper. The more these conditions are 

 fulfilled the larger will be the accumulation at the surface 

 both of nitrogen and ash constituents, and the greater 

 consequently the increased fertility of the soil. 



Leguminous crops, as already mentioned, have a special 

 power of accumulating nitrogen in the surface soil, and 

 are hence of the greatest value in a rotation. Red clover 

 is the most striking instance of this action. Its roots 

 extend further perhaps than those of any other farm crop, 

 and being biennial it has a long period for growth. The 

 accumulation of nitrogen at the surface in the form of 

 roots, stubble, and decayed vegetable matter, is in the case 

 of a good crop of clover so considerable, that the whole of 

 the above ground growth may be removed as hay, and the 

 land yet remain greatly enriched with nitrogen, and in an 

 excellent condition for producing a crop of wheat. 



The ploughing in of green crops has some advantages 

 over the feeding of crops on the land. By this mode of 

 proceeding the whole of the crop is returned to the soil, 

 whereas in feeding a small part of the nitrogen and ash 

 constituents is retained by the animal. The characteristic 

 advantage of green manuring lies, however, in the large 

 amount of humus which the soil acquires. All the carbon 

 which the crop has obtained from the atmosphere is in 

 this case incorporated with the soil, instead of being con- 

 sumed by the animal. Green manuring is especially 



